In the practice of thermal treatment of profiles of rubber or synthetic material, the poor thermal conductivity of such material causes appreciable difficulties in attempting to equalize the heat distribution throughout the profiles since the externally applied heat penetrates but slowly to the core of the workpiece.
In recognition of these difficulties, microwave energy has been used to make possible uniform heating of the profile.
Rectangular waveguides, operating in the H.sub.10 (or TE.sub.10) mode, or circular waveguides, operating in the E.sub.01 (or TM.sub.01) mode, are only limitedly useful for this purpose, though in the vulcanization or cross-linking of polymeric profiles somewhat satisfactory results can be achieved with waveguides of the latter type if the profile to be treated does not surpass a predetermined thickness, up to a diameter of about 30 mm in the case of round workpieces.
It is, however, important in practice to be able to apply such vulcanization and cross-linking also to larger polymeric profiles, e.g. of a cross-section of about 50 .times. 50 mm or more, for which the known teachings are unsuitable.